How to declare a function object in JS?

Question

JavaScript

An obfuscated js file came to the mail, in fact a virus loader. It became interesting what it does, I decided to debug using script.google.com, it's safer.
All work in the script goes through the ActiveX object. Clearly, Google does not have such an object, but it is to our advantage, we will announce our own. Classes could not be used, so through the function.

function cFields() {
  this.append = function(a,b,c){Logger.log('ActiveXObject ('+this.fileName+'.fields) append('+a+','+b+','+c+')');return 'fields.append'};
}

function ActiveXObject(file_name) {
  this.fileName = file_name;
  Logger.log('ActiveXObject ('+this.fileName+') NEW');
  this.Status = 200;
  this.fields = new cFields();
  
  this.open = function(a,b,c){Logger.log('ActiveXObject ('+this.fileName+') open: ('+a+','+b+','+c+')');return 'open'};
  this.send = function(a,b,c){Logger.log('ActiveXObject ('+this.fileName+') send()');return 'send'};
  this.GetSpecialFolder = function(a){Logger.log('ActiveXObject ('+this.fileName+') GetSpecialFolder('+a+')'); return 'SpecFolder'};
  this.GetTempName = function(a){Logger.log("ActiveXObject ("+this.fileName+") GetTempName()"); return 'TempName'};
  this.Open = function(){Logger.log('ActiveXObject ('+this.fileName+') Open()');return 'Open'};
  this.Write = function(a){Logger.log('ActiveXObject ('+this.fileName+') Write('+a+')');return 'Write'};
  this.Read = function(a){Logger.log('ActiveXObject ('+this.fileName+') Read('+a+')');return 'Read'};
  this.addNew = function(a){Logger.log('ActiveXObject ('+this.fileName+'.fields) addNew('+a+')');return 'fields.addNew'};
}

In such a simple way in the logs I see everything that the script does. At some point, the script stops:

TypeError: [object Object] is not a function but an object.

uKi = new ActiveXObject('ADODB.Recordset')
St = 'appendChunk'
q = 'Read'
uKi(Z)[St](q);

Actually the question is how to declare a method in the source object that will be called when the object is accessed like a function?
UPD:
At this link you can see the full script yourself.
https://script.google.com/d/1IrkYN_Sg0j_uiMD4fAmEP...
To launch, execute the doGet function or follow the link:
https://script.google.com/macros/s/AKfycbyiYp4kX07...

Answer 1

So add it.

this.appendChunk = function(a){Logger.log('ActiveXObject ('+this.fileName+') appendChunk('+a+')');return 'appendChunk'};

Answer 2

like so

function ActiveXObject() {
 // ...
  return Object.assign(() => {
    // ...
  }, this);
}

Answer 3

Actually, functions are objects.

function func() {
    alert("Hello world");
}

func.prop1 = "prop";
func.prop2 = "prop";

func(); // Hello world
console.warn(func.prop1); // prop